If you own a DSLR camera, you've undoubtedly seen lenses referred to by their actual focal length and then their 35mm equivalent. One more level of complexity in an already complex world, especially when this extra bit of math so often goes unexplained.
In the old days when real men shot film and real film was 35mm, the size of a single frame was 24x36mm. Yeah, I know--it doesn't seem right that a frame of 35mm film is 36mm. Get over it.
The image your lens puts on that single frame of film is the benchmark by which all these comparisons are made. There are a few DSLRs whose sensor is about that same size as 35mm film. These cameras are usually called "full frame," sit at the top of their respective manufacturer's product lines and cost a lot of money.
If you're shooting with more of a 'prosumer' camera the sensor is smaller. How much smaller is the deciding parameter in the crop factor. My old Canon Digital Rebel 300D, my new Rebel Xsi 450D and the whole Rebel line have a crop factor of 1.6. The mid-range Nikons and a few others have a 1.5 crop factor. The Olympus line's crop factor is 2. The higher the crop factor, the smaller the sensor. Normally the sensor's size tells you a lot about light sensitivity and noise. Size matters. Bigger is normally better.
The size of the sensor also decides how much of the image passed by the lens will be captured. As a smaller portion of what the lens sees is used the resultant image starts resembling what is seen on a full frame camera with a longer lens. A 50mm lens with a cop factor of 1.6 acts like an 80mm lens. With a crop factor of 1.5 the 50mm lens is equivalent to 75mm on a 35mm or full frame camera . At a crop factor of 2 your 50mm shoots like 100mm. These smaller sensors make your telephoto lens act like it's longer so you get more reach for the buck. It also makes it nearly impossible to get a really wide angle lens on a DSLR with a crop factor. That's a big deal. At least to me it is.
Manufacturers have started to take advantage of these smaller sensor sizes by producing lenses that deliver an image just the right fit for the crop factor, though too small to use on a full frame camera. These lenses are smaller, lighter and usually cheaper.
I'm just scratching the surface. There are a few really good reference works at Digital Photography School including "Crop Factor Explained" and "Full Frame Sensor vs Crop Sensor - Which is Right For You?" Meanwhile, keep the crop factor in mind when you're deciding which lens to use or buy.
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August 22, 2008 8:53 AM
good description (of crop factor). I'd also be interested in the point and shoot sensor size and what the impact of the new std re 4/3 sensors is about.
August 22, 2008 1:05 PM
"At a crop factor of 2 your 50mm shoots like 100mm. These smaller sensors make your telephoto lens act like it's longer so you get more reach for the buck."
This is a good example of marketing "math".
Yes, you'll get the same image area as what 100mm sees at that distance. The "equivalent".
But you will get that image with the resolution of 50mm. You will not get the details that 100mm can resolve on that distance.
So, on cropped sensors 50mm lens keeps to act like 50mm. It just gives you cropped image area if you compare the image size against full frame sensor. In fact you'll get LESS IMAGE against full frame with the same lens.
And so, no magic for the buck. Just marketing "math".
August 22, 2008 4:42 PM
Arthur is correct--though possibly not to the extent a totally literal reading of his post would imply (see Pogue's seminal Breaking the Myth of Megapixels for a little more insight).
As I said concerning sensors, "Size matters. Bigger is normally better." Alas, for most of us it comes down to a matter of cash.
Comments correcting/educating this blogger are always welcome.
Geoff Fox
August 23, 2008 7:00 AM
Yes, the sensors keep getting smaller, the prices cheaper, and the quality higher. I found http://www.dslrcamerareviews.com to be a good site to learn more about DSLR cameras,
September 8, 2008 3:27 PM
As a semi-pro photographer and a user of 'full-frame' and 'crop frame' cameras,I have to completely concur with ARTHUR.
"At a crop factor of 2 your 50mm shoots like 100mm. These smaller sensors make your telephoto lens act like it's longer so you get more reach for the buck."
I really wish people who write editorials like this would do their bloody homework.
The crop factor, as wrongly stated above, refers to the FIELD OF VIEW CROP FACTOR of the sensor and is NOT, I repeat NOT, a magnification multiplier for the lens.
These smaller sensors DO NOT make your telephoto lens act like its longer and you are a complete bloody fool if you think that you are getting 'more reach for your buck.
A 50mm lens on a camera body with a 1.6x sensor does not have the telephoto reach of an 80mm lens it has the same (smaller or cropped) field of view of an 80mm lens.
Please get your facts right before publishing crap like this. There are actually people out there who take ludicrous statements like this at face value and actually believe what is written.
This is ostensibly why manufacturers like Canon provide a range of EF-S lenses for APC sensor cameras to compensate for this FILED OF VIEW CROP FACTOR.